Urinary Concentrating Mechanism in the Dog

Abstract

The osmotic U/P ratio has been detd. 11 times in 7 dogs, following 48 hrs. deprivation of food and water. Values ranged from 2.51-5.81 and averaged 4.45. Graded osmotic diuresis was induced with mannitol under conditions of dehydration 8 times in 2" dogs with artificial bladder extrophies, collections always being taken from the left ureter only. Pitressin was given throughout the expts. to assure max. osmotic concn. As the quantity of solute destined for excretion (C osm.) increases, the quantity of water reabsorbed v/ithout solute (i.e., the "concentrating mechanism") TCH2O, increases to reach an approx. constant, max. value. These results are compatible with the theory that, given a sufficient distal load of water (and solutes), renal osmotic concn. under conditions of max. ADH activation is effected by the reabsorption of a small, constant quantity of water from an isosmotic tubular fluid.